How Due Dates Are Calculated: LMP, Ultrasound, and Why Your Estimate May Change
The estimated due date is one of the most important numbers in pregnancy care — and one of the most misunderstood. Most people know it comes from the last period, but few understand exactly how the calculation works, when and why it might change, or how accurate it really is.
This guide covers the standard methods used to calculate due dates, how they compare in accuracy, the conditions under which providers revise estimates, and what the due date actually represents. If you want to calculate your own estimate right now, the due date calculator does this instantly from your LMP or an approximate conception date.
280
Days from LMP to due date
40 weeks standard
266
Days from conception
38 weeks from fertilization
±5–7
Days accuracy
first-trimester ultrasound
~5%
Born on exact due date
most within ±2 weeks
The Standard Method: LMP and Naegele's Rule
The most widely used method for calculating a due date is Naegele's rule, developed in the 19th century and still the default in clinical practice. It works by taking the first day of the last menstrual period and adding 280 days, which equals 40 weeks.
The logic behind 280 days is rooted in the medical convention of counting gestational age from the LMP rather than from conception. In a typical 28-day cycle, ovulation happens around day 14, meaning conception is roughly 2 weeks after the LMP start. Adding 266 days from conception to the due date, plus those 2 weeks, produces the 280-day total. This is why the due date is described as 40 weeks gestational age but only about 38 weeks from actual fertilization.
Naegele's rule assumes a 28-day cycle and ovulation on day 14. For people with longer or shorter cycles, calculators adjust the estimate accordingly. The due date calculator includes a cycle length adjustment for this reason.
The Four Dating Methods Compared
LMP Method (Naegele's Rule)
Add 280 days (40 weeks) to the first day of the last menstrual period. The standard method used by virtually all providers as the starting estimate.
First-Trimester Ultrasound
Crown-rump length (CRL) measured between weeks 7–13 is converted to gestational age using established growth charts. Most accurate dating method.
Second-Trimester Ultrasound
Multiple fetal measurements (BPD, femur length, abdominal circumference) combined into a composite gestational age estimate.
Conception Date (Known)
Add 266 days (38 weeks) to a confirmed conception date. Useful after IVF or closely tracked ovulation cycles.
When and Why Your Due Date Might Change
A due date change after an ultrasound is one of the most common sources of confusion. The American College of Obstetricians and Gynecologists (ACOG) has specific guidelines about when ultrasound findings should override the LMP-based estimate.
In the first trimester, if the ultrasound-based gestational age differs from the LMP-based estimate by more than 5 to 7 days (the exact threshold depends on the gestational age at the time of the scan), providers typically redatate the pregnancy to match the ultrasound. First-trimester crown-rump length measurement is the most accurate dating tool available — more accurate than LMP in most cases.
In the second trimester, the threshold for redating is wider — up to 10 to 14 days — because fetal growth variation increases naturally as the pregnancy progresses. Redating at this stage is done more cautiously. By the third trimester, due dates are rarely changed based on ultrasound alone because growth variation between fetuses is too large to use as a reliable dating reference.
What the Due Date Actually Represents
Despite the name, a due date is not a precise delivery prediction. It is a statistical midpoint — the center of a normal distribution of expected delivery times. In a population of pregnancies, the average delivery occurs close to 40 weeks, but individual variation around that average is wide.
Research suggests that only about 4 to 5 percent of births occur exactly on the estimated due date. The vast majority of full-term births fall within the two-week window from 39 to 41 weeks. Medically, births from 37 weeks onward are generally considered in the range of acceptable term delivery, though 39 to 40 weeks is associated with the best outcomes.
The practical implication is that the due date is useful for organizing the pregnancy timeline — scheduling appointments, knowing when screenings are appropriate, and planning in general — but it should not be treated as a guaranteed event date. Use the pregnancy countdown calculator to track the remaining days without over-weighting the exact date.
Common Misconceptions About Due Dates
Myth: My due date is when I will give birth.
Fact: Only about 5% of babies are born on the exact estimated due date. Most births happen within a two-week window on either side of it.
Myth: A later scan overrides my original due date.
Fact: Not necessarily. ACOG guidelines recommend redating only when the discrepancy between LMP and ultrasound exceeds specific thresholds (e.g., >5 days in the first trimester). Ask your provider if a date change is being considered.
Myth: If my cycle is irregular, my due date is meaningless.
Fact: Irregular cycles make the LMP method less reliable, but a first-trimester ultrasound can establish a more accurate due date directly from fetal measurements regardless of cycle length.
Myth: A due date of 40 weeks means the baby is overdue before then.
Fact: The due date is the midpoint of the full-term window, not a deadline. Births from week 39 through 41 are all considered in the normal full-term range.
Due Date Calculator
Calculate your estimated due date from LMP or conception date.
How Many Weeks Pregnant Am I?
Find your current gestational age from your LMP or due date.
Pregnancy Countdown
See exactly how many days remain until your due date.
Conception Date Calculator
Estimate conception timing from your due date or current week.
Frequently Asked Questions
Can my due date change after an ultrasound?
Yes, if the ultrasound measurement differs from the LMP-based estimate by more than the threshold for that gestational age. First-trimester scans are the most likely to produce a date change. Second-trimester redating is done more cautiously, and third-trimester redating is rare.
What if I don't know my LMP?
A first-trimester ultrasound can establish gestational age from fetal measurements without needing the LMP. If you have an approximate conception date — from ovulation tracking or fertility treatment — the conception method can also provide an estimate.
Is a due date of 40 weeks the same across all methods?
The due date is defined as 40 weeks gestational age, regardless of the dating method. What changes between methods is the confidence in where 40 weeks actually falls. First-trimester ultrasound generally places that point more precisely than LMP alone.
Do irregular periods affect due date accuracy?
Yes. Irregular cycles make the LMP-based estimate less reliable because the assumption of day-14 ovulation does not hold. In these cases, a first-trimester ultrasound is the most accurate way to establish gestational age and set a reliable due date.